The brushless permanent magnet electric motors preferably used to drive compressors comprise a rotor with a plurality of permanent magnets mounted to a rotatable shaft that is driven by a stator comprising a plurality of windings adjacent, and preferably surrounding, the permanent magnets. The electric motor is powered through an electronic controller that generates polyphase alternating current for application to the stator winding and the creation of a rotating magnetic field that interacts with and rotates the permanent magnets and motor shaft. The components of the electronic controller include temperature sensitive semiconductor devices, such as MOSFETS, to convert direct current power into the polyphase alternating current necessary for operation of the permanent magnet motors, as disclosed, for example, in U.S. patent application Ser. No. 08/782,566 filed Jan. 10, 1997, now U.S. Pat. No. 5,841,649. Though efficient, such semi conductor devices generate significant heat loss in their operation, particularly when called upon to control the application of high currents to the motor stator windings.
Motor-driven compressors currently in use in commercial internal combustion engines, shown in FIG. 1, usually consist of such brushless electric motors mounted in an aluminum housing and driving a centrifugal air compressor wheel within an enclosing compressor casing. Such motor-driven compressors are frequently installed within the engine compartment of a vehicle, as shown in FIG. 2, where the surrounding environment is at a substantially elevated temperature. In the operation of such air compressors, their motors are energized from a power source such as a battery through an electronic controller, which, as described above, changes direct current from the battery to polyphase alternating current to produce a rotating magnetic field in the motor windings. The rotating field interacts with motor magnets mounted on the drive shaft and generates torque that rotates the compressor wheel and shaft assembly. The compressor wheel induces air from the atmosphere, generally through an air cleaner, into the compressor air inlet and delivers it from the compressor casing at above-atmospheric pressure.
In many applications, such as supercharging systems for internal combustion engines, small size is an advantage. Smaller compressors, however, require higher motor operating speeds to provide sufficient compressed air. Because, among other things, the compressor motor losses are concentrated in, and must be dissipated from, the smaller compressor housing, the compressor motor becomes more temperature sensitive. By "temperature sensitive," we mean a motor or electronic component whose reliability may be at risk, or whose performance may be degraded by the inability to dissipate heat generated during its operation. Motor-driven compressor systems have been improved by bleeding a small portion of the compressed air from the compressor to flow through the motor housing as cooling air for the motor windings, as disclosed in U.S. patent application Ser. No. 08/926,881 filed Sep. 10, 1997.
As set forth above, electronic controllers for high-speed compressor motors include temperature sensitive components, and must be cooled accordingly. Controller housings are typically cooled with a multiplicity of external notches or fins that transfer heat generated by the electronics to the atmosphere. In internal combustion engine applications, compressor motor electronic controllers are also frequently placed at a location remote from the compressor motor to avoid exposure to elevated environmental temperatures (see FIG. 2).
Low-speed compressor motors have employed internal fans attached to their shafts to produce a flow of cooling air through the motor and around its internal components to reduce their temperatures. However, with very highspeed brushless motors, the use of small internal fans imposes significant and undesirable loads on the electric motor, and complicates the internal construction of the compressor housing assembly.
Consequently, compromises in the selection and use of electric motors have been required because of hostile motor environments and the relative inefficiencies of small motors. In addition, the temperatures generated in their electronic controllers have frequently limited the amount of compressed air that can be reliably produced on a continuous basis by current motor-driven compressors.